A nuclease has been purified from a marine Pseudomonas strain which breaks a DNA chain everywhere a single-strand scission occurs, attacks supercoiled closed circular DNA to yield linear duplex molecules, and shortens linear duplex molecules in the manner expected of a double-strand exonuclease. Single-stranded DNA is degraded to 5' mononucleotides and the intermediate products of exonuclease digestion of duplex DNA are also mononucleotides. Closed circular duplex DNA's containing very few superhelical turns are not attacked. An RNase activity is present as well. All these activities (except for the RNase activity, which has not yet been examined) are found in a single band of protein in gel electrophoresis experiments. The sedimentation coefficient, molecular weight, optimal pH and temperature for activity, required concentrations of magnesium and calcium ions (required cofactors) for maximal activity, and kinetic parameters (Michaelis constants, etc.) have been determined. The kinetic data have been obtained for the activity against denatured DNA and for the double-strand exonuclease activity. Monitoring of these activities in the presence of high concentrations of salts or agents which often denature proteins has revealed a remarkable stability for this nuclease. It is active in the presence of 4.5 NaCl, 6 M CsCl, 6 M urea, and 5 percent sodium dodecyl sulfate. If low levels of certain types of lesions, which do not introduce nicks into the DNA at a substantial level, are introduced in non-supercoiled closed circular bacteriophage PM2 DNA, this DNA clearly become susceptible to the enzyme as shown in studies currently in progress. Some examples of such lesions are those introduced by ultraviolet irradiation (presumably pyrimidine dimers), a potent carcinogen (aminofluorene derivative) and cross-links introduced with nitrous acid.